Lithium secondary battery

ABSTRACT

Provided is a lithium secondary battery containing a cathode active material capable of preventing decreases in power and cycle life occurring at the time of adding a sulfur based additive used in order to improve high-temperature storage characteristics to an electrolyte.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/386,373 filed on Dec. 21, 2016, which claims benefits of priority ofKorean Patent Application No. 10-2016-0004000 filed on Jan. 13, 2016.The disclosure of each of the foregoing application is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The following disclosure relates to a lithium secondary battery of whicha trade-off of power and cycle life characteristics is significantlydecreased and high-temperature storage characteristics are significantlyimproved by using a cathode active material having a concentrationgradient and a non-aqueous electrolyte containing a sulfur basedadditive.

BACKGROUND

A cathode active material used in a battery is an important portion forimproving performance of the battery, and particularly, in order tomanufacture a battery having a high-energy density and high powerperformance, a high-capacity cathode active material is required.

In addition to improving performance of the battery through thehigh-capacity cathode active material, in order to improve storagecapability of the battery at a high temperature, a sulfur based additiveis mainly added to and used in an electrolyte. However, in the case ofusing the sulfur based additive, a cycle life of the battery isdecreased, and power characteristics of the battery are deteriorated.Therefore, there is a need to solve problems occurring at the time ofusing the sulfur based additive while using the high-capacity cathodeactive material to improve performance of the battery.

The development of novel technology capable of improving performance ofthe battery while hardly causing a trade-off of power and cycle lifecharacteristics in spite of using the sulfur based additive as describedabove has been urgently demanded.

For example, a lithium battery using an electrolyte containing a solidsulfide, in which a specific metal element concentration is high in aportion contacting a solid electrolyte has been disclosed in U.S. PatentApplication Publication No. 2013-0065135. However, in most studies,these technologies are to improve performance of the battery itself, andthe development of a technology for cathode active material capable ofovercoming disadvantages of the lithium battery using the electrolytecontaining the sulfur based additive was not reported.

RELATED ART DOCUMENT Patent Document

(Patent Document 1) U.S. Patent Application Publication No. 2013-0065135

SUMMARY

An embodiment of the present invention is directed to providing abattery capable of solving problems of a lithium ion secondary batteryoccurring due to a sulfur based electrolyte additive and capable ofhaving significantly excellent high-temperature storage capability andcycle life characteristics by using the sulfur based electrolyteadditive and a cathode active material having a concentration gradientformed in a predetermined region in a thickness direction of the cathodeactive material as a cathode material.

In one general aspect, a lithium secondary battery contains a cathode,an anode, and a non-aqueous electrolyte, wherein the non-aqueouselectrolyte contains a sulfur based. additive, and the cathode containscathode active material containing a lithium-metal oxide particleforming a concentration gradient. The concentration gradient typecathode active material may be a cathode active material in which aconcentration of a metal is changed between a surface part and a corepart of the lithium-metal oxide particle, and the present inventionprovides the lithium secondary battery containing the same.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a lithium-metal oxide particle usedin Examples 1 to 24 and Comparative Examples 1 to 26, and schematicallyillustrates concentration measurement positions of a metal element.

FIG. 2A, is a schematic cross-sectional view of the lithium-metal oxideparticle used in Examples 1 to 24 and Comparative Example 1, and FIG. 2Bis a cross-sectional view enlarged based on the concentration gradientlayer and illustrates concentration measurement positions of a metal ofthe lithium-metal oxide particle used in Examples 1 to 24 andComparative Example 1.

FIG. 3A illustrates a change in Ni concentration in the lithium-metaloxide particle used in Examples 1 to 24 and Comparative Example 1, andFIG. 3B illustrates a change in Mn concentration in the lithium-metaloxide particle used in Examples 1 to 24 and Comparative Example 1.

FIG. 4 is a cross-sectional photograph of the lithium-metal oxideparticle used in Examples 1 to 24 and Comparative Example 1.

FIG. 5 is a cross-sectional photograph of the lithium-metal oxideparticle used in Comparative Examples 2 to 26.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, the present invention will be described in detail. Adescription for the known function and configuration obscuring thepresent invention will be omitted in the following description and theaccompanying drawings. Unless defined otherwise in the specification, itis to be understood that all terms used in the specification areconstrued as meaning as those that are generally used in the art.

The present invention, which relates to a battery having improvedperformance while containing a sulfur based additive in an electrolyte,relates to a lithium secondary battery capable of solving problems suchas a decrease in power of the battery, or the like, occurring due to thesulfur based additive, and having excellent cycle life characteristics.

The present invention relates a lithium secondary battery including acathode, an anode, and a non-aqueous electrolyte, wherein thenon-aqueous electrolyte contains a sulfur based additive, and thecathode contains a cathode active material containing a lithium-metaloxide particle, the cathode active material having a concentrationgradient layer in which a concentration of a metal contained in thelithium-metal oxide particle is changed between a surface part and acore part of the lithium-metal oxide particle.

In the present invention, the cathode active material is thelithium-metal oxide particle, the core part may mean a region from thereal center of the lithium-metal oxide particle to a portion thereofhaving a radius, in which the concentration or composition of the metalcontained in the lithium-metal oxide particle is constant, and thesurface part may mean a region from an outermost portion of thelithium-metal oxide particle to an inner portion thereof, in which theconcentration or composition of the metal contained in the lithium-metaloxide particle is constant. The concentration gradient is formed betweenthe core part and the surface part of the cathode active material. Inthe present invention, the meaning of the constant concentration orcomposition includes a concentration or composition included in an errorrange acceptable in the art to which the present invention pertains. Forexample, in the present invention, the case in which an error in molarratio of the metal contained in the lithium-metal oxide particlecorresponding to the cathode active material contained in the lithiumsecondary battery is in a molar ratio range of ±0.001 to ±0.003 may beconsidered as the constant concentration or composition.

A shape of the cathode active material contained in the lithiumsecondary battery according to the present invention will be describedin more detail with reference to FIGS. 1 and 2. FIG. 1, which is across-sectional view of the cathode active material according to thepresent invention, illustrates sections denoted numbers 1 to 13 betweenthe core part of the cathode active material and the outermost portionthereof, divided depending on a concentration measurement position andrange. The center of the section denoted by number 1 is a real center,and the section denoted by number 13 contacts the outermost portion.Referring to FIG. 1, sections denoted by numbers 1 to 12 may correspondto the core part, and the sections denoted by numbers 2 to 13 maycorrespond to the surface part. For example, the section denoted bynumber 1 corresponds to the core part and sections denoted by numbers 2to 13 correspond to the surface part; the sections denoted by numbers 1to 2 correspond to the core part, and the sections denoted by numbers 3to 13 correspond to the surface part; the sections denoted by numbers 1to 11 correspond to the core part, and the sections denoted by numbers12 to 13 correspond to the surface part; or the sections denoted bynumbers 1 to 12 correspond to the core part, and the section denoted bynumber 13 corresponds to the surface part, etc. The core part and thesurface part of the cathode active material particle may be divided asdescribed above, and the concentration gradient layer of the metal isformed between the core part and the surface part. In more detail,according to an exemplary embodiment of the present invention, the casein which the core part is a region from the center of the sectiondenoted by number 1 to the center of the section denoted by number 12,the concentration gradient layer is formed in a portion between acentral portion of the section denoted by number 12 and the center ofthe section denoted by number 13, and the surface part is a region froma portion of the section denoted by number 12 at which a concentrationgradient is terminated to the section denoted by number 13 may beillustrated as in FIG. 2, and a concentration gradient of one or moremetals contained in the lithium-metal oxide particle, the cathode activematerial, may be formed in the concentration gradient layer. Thedescription of the cathode active material described with reference toFIGS. 1 and 2 is to assist in understanding of the present invention,and the present invention is not limited thereto.

According, to the exemplary embodiment of the present invention, aportion in which a concentration gradient of the metal is continuouslyor traditionally formed between the core part and the surface part ofcathode active material may be included in the cathode active material.The concentration gradient as described above may significantly improvehigh-temperature storage characteristics while minimizing decreases inpower and cycle life of a lithium battery, which may occur due to thesulfur based additive contained in the non-aqueous electrolyte.

The cathode active material contained in the cathode of the lithiumsecondary battery, which is a lithium-metal oxide having theconcentration gradient layer, may improve performance of the battery.Particularly, even in the case of using the sulfur based additive in theelectrolyte, problems such as decreases in cycle life and power due tothe sulfur based additive hardly occur, and storage characteristics aresignificantly improved, thereby making it possible to manufacture asecondary battery having balanced performance.

According to the present invention, the cathode active material containsthe lithium-metal oxide particle, and at least one metal contained inthe lithium-metal oxide particle except for lithium forms theconcentration gradient layer in which a concentration of the metal ischanged between the surface part and the core part. The concentrationgradient layer means a layer having concentration distribution in whichthe concentration of the metal is changed in a constant trend betweenthe surface part and the core part. Here, the constant trend may be acontinuous or traditional change in concentration occurring in apredetermined section between the surface part and the core part.

In the present invention, the lithium-metal oxide particle is notlimited as long as it is used in a lithium battery, but may be, forexample, represented by the following Chemical Formula 1.Li_(x)M1_(a)M2_(b)M3_(c)O_(y)   [Chemical Formula 1]

(In Chemical Formula 1, M1, M2, and M3 are each independently differentmetals selected from the group consisting of Ni, Co, Mn, Na, Mg, Ca, Ti,V, Cr, Cu, Zn, Ge, Sr, Ag, Ba, Zr, Nb, Mo, Al, Ga, B, and a combinationthereof, and x, y, a, b, and c satisfy 0<x≤1.1, 2≤y≤2.02, 0≤a≤1, 0≤b≤1,0≤c≤1, and 0<a+b+c≤1.)

In the lithium-metal oxide particle, a concentration of at least onemetal of M1, M2, and M3 in Chemical Formula 1 is changed between thesurface part and the core part. That is, the lithium-metal oxideparticle contained in the cathode active material may contain at leastone metal having a concentration gradient.

According to the exemplary embodiment of the present invention,concentration gradients of different metals contained in thelithium-metal oxide particle may be contrary to each other. In addition,another metal having a constant concentration between the surface partand the core part may be further contained in the lithium-metal oxideparticle together with the metals having, concentration gradientscontrary to each other. The metals having concentration gradientscontrary to each other may be represented by first and third metals M1and M3, and it is preferable that the metals having concentrationgradients contrary to each other are each independently one or moredifferent metals. The metal having a constant concentration may berepresented by a second metal M2, and it is preferable that the secondmetal M2 is one or more metals different from the first and third metalsM1 and M3.

According to the present invention, the lithium-metal oxide particlerepresented by Chemical Formula 1 may have a concentration gradientlayer of the metal M1 in which a concentration of the metal M1 isdecreased in a surface direction between the surface part and the corepart, contain a constant concentration of the metal M2 between surfacepart and the core part, and have a concentration gradient layer of themetal M3 in which a concentration of the metal M3 is increased in thesurface direction between the surface part and the core part.

In the present invention, the metals contained in the lithium-metaloxide particle, the cathode active material contained in the lithiumsecondary battery, are not limited as long as they are used in thelithium battery, but may be, for example, Ni, Co, and Mn, and may berepresented by the following Chemical Formula 2.Li_(x)Ni_(a)Co_(b)Mn_(c)O_(y)   [Chemical Formula 2]

(Here, x, y, a, b, and c satisfy 0<x≤1.1, 2≤y≤2.02, 0≤a≤1, 0≤b≤1, 0≤c≤1,and 0<a+b+c≤1.)

According to the exemplary embodiment of the present invention, in thecase in which the metals contained in the lithium-metal oxide particleare Ni, Co, and Mn, and are represented by Chemical Formula 2, thelithium-metal oxide particle may have a concentration gradient layer ofa Ni metal in which a concentration of the Ni metal is decreased in thesurface direction between the surface part and the core part, contain aconstant concentration of a Co metal between the surface part and thecore part, and have a concentration gradient layer of a Mn metal inwhich a concentration of the Mn metal is increased in the surfacedirection between the surface part and the core part.

The lithium secondary battery using the cathode active material in whichthe concentration of the metal contained in the lithium-metal oxideparticle is changed as described above has excellent cycle lifecharacteristics as compared to a lithium secondary battery using acathode active material in which a concentration of a metal is notchanged, and containing a sulfur based additive.

A concentration range of the metal in the lithium-metal oxide particleused in the present invention may be adjusted but is not particularlylimited.

For example, according to the exemplary embodiment of the presentinvention, in Chemical Formula 2, a range of a capable of indicating achange in concentration of Ni may satisfy 0.60≤a≤0.95, 0.70≤a≤0.90,preferably 0.75≤a≤0.85, but is not limited thereto. According to theexemplary embodiment of the present invention, in Chemical Formula 2, arange of c capable of indicating a change in concentration of Mn maysatisfy 0.065≤c≤0.140, 0.075≤c≤0.135, 0.080≤c≤0.130, preferably0.085≤c≤0.125, but is not limited thereto. According to still anotherexemplary embodiment of the present invention, in Chemical Formula 2, asum (b+c) of the concentrations of Co of which the concentration isconstant and Mn of which the concentration is increased between thesurface part and the core part may satisfy 0.050≤b+c≤0.400,0.100≤b+c≤0.350, 0.100≤b+c≤0.350, 0.150≤b+c≤0.300, preferably0.200≤b+c≤0.250, but is not limited thereto.

According to another embodiment of the present invention, a lithiumsecondary battery includes a cathode, an anode, and a non-aqueouselectrolyte, wherein the non-aqueous electrolyte contains a sulfur basedadditive, and the cathode contains a lithium-metal oxide particle, thelithium-metal oxide particle, which is a cathode active material,including a core part represented by Chemical Formula 3 and a surfacepart represented by Chemical Formula 4, a concentration of M1 beingdecreased in a surface direction between the core part and the surfacepart, and a concentration of M3 being increased in the surface directionbetween the core part and the surface part.Li_(x2)M1_(a2)M2_(b2)M3_(c2)O_(y2)   [Chemical Formula 3]Li_(x3)M1_(a3)M2_(b3)M3_(c3)O_(y3)   [Chemical Formula 4]

(In Chemical Formulas 3 and 4, M1, M2 and M3 are each independentlydifferent metals selected from the group consisting of Ni, Co, Mn, Na,Mg, Ca, Ti, V, Cr, Cu, Zn, Ge, Sr, Ag, Ba, Zr, Nb, Mo, Ga, B, and acombination thereof, and x2, x3, y2, y3, a2, a3, b2, b3, c2, and c3satisfy 0<x2≤1.1, 0<x3≤1.1, 2≤y2≤2.02, 2≤y3≤2.02, 0≤a2≤1, 0≤a3≤1,0≤b2≤1, 0≤b3≤1, 0≤c2≤1, 0≤c3≤1, 0<a2+b2+c2≤1, 0<a3+b3+c3≤1, a3<a2, andc2<c3.)

In Chemical Formulas 3 and 4, M2 may be a metal having a constantconcentration between the core part and the surface part.

According to the exemplary embodiment of the present invention, inChemical Formulas 3 and 4, M1 may be Ni, M2 may be Co, M3 may be Mn, thecore part may be represented by Chemical Formula 5, and the surface partmay be represented by Chemical Formula 6.Li_(x4)Ni_(a4)Co_(b4)Mn_(c4)O_(y4)   [Chemical Formula 5]Li_(x5)Ni_(a5)Co_(b5)Mn_(c5)O_(y5)   [Chemical Formula 6]

(In Chemical Formulas 5 and 6, x4, x5, y4, y5, a4, a5, b4, b5, c4, andc5 satisfy 0<x4≤1.1, 0<x5≤1.1, 2≤y4≤2.02, 2≤y5≤2.02, 0.800≤a4≤1.000,0≤a5≤0.770, 0≤b4≤1.000, 0≤b5≤1.000, 0≤c4≤0.090, 0.120≤c5≤1.000,0<a4+b4+c4≤1, and 0<a5+b5+c5≤1.)

In Chemical Formulas 5 and 6, a concentration of Co may be constantbetween the core part and the surface part.

A lithium secondary battery manufactured by using a lithium-metal oxidefor a cathode active material in which a concentration gradient isformed as a cathode material, and adding a sulfur based additive to anon-aqueous electrolyte while using nickel, cobalt, and manganese asmetals contained in the lithium-metal oxide and adjusting compositionratios thereof as in Chemical Formulas 5 and 6 may further minimize atrade-off of power and cycle life characteristics. According to theexemplary embodiment of the present invention, when a range in which aconcentration gradient of Ni is formed is 0.770 to 0.800, and a range inwhich a concentration gradient of Mn is formed is 0.090 to 0.120, aconcentration gradient may not be rapidly formed between the core partand the surface part, the cathode active material may be structurallystable, and an effect of improving characteristics of the lithiumsecondary battery by the sulfur based additive and an effect ofsuppressing the trade-off of the power and cycle life characteristics bythe sulfur based additive may be more excellent.

According to the present invention, it is possible to solve problemssuch as decreases in power and cycle life by the sulfur based additiveadded to the electrolyte, and further improve high-temperature storagecharacteristics which are the reason why the sulfur based additive isadded.

The sulfur based additive added to the electrolyte, which is preferablya sultone based additive, may be 1,3-propane sultone, 1,3-propenesultone, or a mixture thereof. The sulfur based additive used a batteryis an additive improving high-temperature storage characteristics butmay decrease cycle life and power of the battery. However, according tothe present invention, the concentration gradient layer of one or moremetals is formed between the surface part and the core part of thelithium-metal oxide particle, the cathode active material, therebymaking it possible to minimize the trade-off of the power and cycle lifecharacteristics while improving the high-temperature storagecharacteristics by the sulfur based addictive.

A content of 1,3-propane sultone, 1,3-propene sultone, or the mixturethereof, which is the sulfur based additive added to the electrolyte, isnot particularly limited, and may be contained in a range suitable forpreventing a decrease in power of the battery and improving storagecharacteristics and the cycle life of the battery. For example,according to the exemplary embodiment of the present invention, in thecase of using 1,3-propane sultone as the sulfur based additive added tothe electrolyte, 0.1 to 3.0 wt %, 0.5 to 2.5 wt %, 0.5 to 1.5 wt %, andpreferably, 0.5 to 2.0 wt % of 1,3-propane sultone may be contained inthe electrolyte, but the content is not limited thereto. According toanother exemplary embodiment of the present invention, in the case ofusing 1,3-propene sultone as the sulfur based additive added to theelectrolyte, 0.1 to 3.0 wt %, 0.5 to 2.5 wt %, 0.5 to 1.5 wt %, andpreferably, 0.5 to 2.0 wt % of 1,3-propene sultone may be contained inthe electrolyte, but the content is not limited thereto. According toanother exemplary embodiment of the present invention, in the case ofusing the mixture of 1,3-propane sultone and 1,3-propene sultone as thesulfur based additive added to the electrolyte, high-temperature storagecharacteristics and cycle life characteristics may be more excellent. Inthe case of using the mixture of 1,3-propane sultone and 1,3-propenesultone as the sulfur based additive added to the electrolyte, 0.1 to4.0 wt %, 0.1 to 3.5 wt %, 0.1 to 3.0 wt %, 0.1 to 2.5 wt %, 0.5 to 4.0wt %, 0.5 to 3.5 wt %, 0.5 to 3.0 wt %, 0.5 to 2.5 wt %, 1.0 to 4.0 wt%, 1.0 to 3.5 wt %, 1.0 to 2.5 wt %, and preferably 1.0 to 3.0 wt % ofthe mixture of 1,3-propane sultone and 1,3-propene sultone may becontained in the electrolyte, but content not limited thereto. In thiscase, it is possible to more effectively suppress a decrease in cyclelife by the trade-off while improving high-temperature storagecharacteristics.

In the present invention, a shape of the lithium-metal oxide particle isnot particularly limited, but it is preferable that as a primaryparticle becomes close to the surface part of the lithium-metal oxideparticle, the primary particle may have a rod-type shape.

In the present invention, a particle size of the lithium-metal oxide isnot particularly limited, but may be, for example, 3 to 25 μm.

The cathode active material according to the present invention may be alithium-metal oxide further including a coating layer. The coating layermay be formed of a metal or metal oxide, for example, Al, Ti, Ba, Zr,Si, B, Mg, P, and an alloy or oxide thereof.

The cathode active material according to the present invention may be alithium-metal oxide doped with a metal ingredient. For example, a metalcapable of being doped may be Al, Ti, Ba, Zr, Si, B, Mg, P, V, and acombination thereof.

The present invention may provide a lithium secondary batterymanufactured using the above-mentioned cathode active material and theelectrolyte containing the sulfur based additive.

The lithium secondary battery may be manufactured to include a cathode,an anode, and a non-aqueous electrolyte.

The cathode and the anode may be manufactured by mixing and stirring thecathode active material according to the present invention and an anodeactive material with a solvent, if necessary, a binder, a conductivematerial, a. dispersant, and the like, to prepare electrode mixtures,respectively, applying (coating) and drying the electrode mixtures oncurrent collectors of a metal material, and then pressing the appliedelectrode mixtures, respectively.

In the present invention, as the cathode active material, thelithium-metal oxide particle having the concentration gradient suitablefor achieving the object of the present invention may be prepared by amethod known in the art and then used, and a preparation method thereofis not limited.

As the anode active material, an active material generally used in ananode of a secondary battery may be used. As an example, in the lithiumsecondary battery, the anode active material may be a material capableof intercalating lithium. As a non-restrictive example, the anode activematerial may be at least one material selected from the anode activematerial group consisting of lithium (metal lithium) easilygraphitizable carbon, hardly graphitizable carbon, graphite, silicon, aSn alloy, a Si alloy, a Sn oxide, a Si oxide, a oxide, a Ni oxide, a Feoxide (FeO), and lithium-titanium oxides (LiTiO₂, and Li₄Ti₅O₁₂) and bea composite of at least two materials selected from the anode activematerial group.

As the binder, any binder may be used without particular limitation aslong as it is used in the art. For example, an organic binder such as apolyvinyldenefluoride-hexafluoropropylene copolymer (PVDF-co-HFP),polyvinylidenefluoride (PVDF), polyacrylonitrile,polymethylmethacrylate, and the like, or an aqueous binder such asstyrene-butadiene rubber (SBR), or the like, may be used together with athickener such as carboxymethyl cellulose (CMC).

As the conductive material, a general conductive carbon material may beused without particular limitation.

As the current collect of the metal material, any metal may be used aslong as the electrode mixtures of the cathode or anode active materialmay be easily adhered to the metal, and the metal does not havereactivity in a voltage range of a battery. As a non-restrictive exampleof a cathode current collector, there is foil made of aluminum, nickelor a combination thereof, and the like, and as a non-restrictive exampleof an anode current collector, there is foil made of copper, gold,nickel, a copper alloy, or a combination thereof, and the like.

A separator is interposed between the cathode and the anode, and as theseparator, a general porous polymer film used as a separator accordingto the related art may be used. For example, a porous polymer film madeof a polyolefin based polymer such as an ethylene homopolymer, apropylene homopolymer, an ethylene/butene copolymer, an ethylene/hexenecopolymer, ethylene/methacrylate copolymer, or the like, may be usedalone, or these porous polymer films are stacked and then used. Further,general porous non-woven fabric, for example, non-woven fabric made ofhigh-melting point glass fiber, polyethylene terephthalate fiber, or thelike, may be used. As a method of applying the separator to the battery,a method of laminating (stacking) the separator and electrodes, afolding method, and the like, as well as a winding method, which is ageneral method, may be used.

In the present invention, the non-aqueous electrolyte contains a lithiumsalt, an electrolyte, and an organic solvent, and as the lithium salt,any lithium salt may be used without limitation as long as it isgenerally used in an electrolyte for a lithium secondary battery, andmay be represented by Li⁺X⁻.

An anion of the lithium salt is not particularly limited, but any one ormore selected from F⁻, Cl⁻, Br⁻, I⁻, NO₃ ⁻, N(CN)₂ ⁻, BF₄ ⁻, ClO₄ ⁻, PF₆⁻, (CF₃)₂PF₄ ⁻, (CF₃)₃PF₃ ⁻, (CF₃)₄PF₂ ⁻, (CF₃)₅PF⁻, (CF₃)₆P⁻, CF₃SO₃ ⁻,CF₃CF₂SO₃ ⁻, (CF₃SO₂)₂N⁻, (FSO₂)₂N⁻, CF₃CF₂(CF₃)₂CO⁻, (CF₃SO₂)₂CH⁻,(SF₅)₃C⁻, (CF₃SO₂)₃C⁻, CF₃(CF₂)₇SO₃ ⁻, CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻, and(CF₃CF₂SO₂)₂N⁻may be used.

As the, organic solvent, any one or a mixture of two or more selectedfrom the group consisting of propylene carbonate (PC), ethylenecarbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC),ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate,dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane,sulfolane, gamma-butyrolactone, and tetrahydrofuran may be used.

The non-aqueous electrolyte is injected into an electrode structurecomposed of the cathode, the anode, and the separator interposed betweenthe cathode and the anode.

There is no particular limitation in an outer shape of the lithiumsecondary battery, but the lithium secondary battery may be acylindrical or prismatic battery using a can, a pouch-type or coin-typebattery, or the like.

Hereinafter, the present invention will be described in detail throughExamples. The following Examples are described in order to assist inunderstanding the exemplary embodiment of the present invention, but thepresent invention is not limited thereto.

Evaluation of Battery Characteristics

1. Power Characteristics

Power characteristics of batteries manufactured in Examples andComparative Examples were measured according to hybrid pulse powercharacterization (HPPC) by FreedomCar battery test manual.

2. High-Temperature Storage Characteristics

After keeping batteries manufactured in Examples and ComparativeExamples in a high temperature chamber (60° C.) for 4 weeks in a SOC100charge (1C 4.2V 0.1C CUT-OFF) state and keeping the batteries at roomtemperature for 12 hours, recovery charge capacity thereof was measuredunder the same 1C condition.

3. Cycle Life Characteristics

After repeating a charging operation (1C 4.2V 0.1C CUT-OFF) and adischarging operation (1C 3.0V CUT-OFF) 500 times using the batteriesmanufactured in Examples and Comparative Examples, a percentage of avalue obtained by dividing discharge capacity at 500^(th) cycle bydischarge capacity at 1^(st) cycle was illustrated.

EXAMPLES 1 to 24

Manufacturing of Cathode

As a cathode active material, lithium-metal oxides (hereinafter, CAM10)of which an entire composition was LiNi_(0.80)Co_(0.11)Mn_(0.09)O₂, acomposition of a core part was LiNi_(0.802)Co_(0.11)Mn_(0.088)O₂ (Numberin Table 1: 1 to 12), a composition of a surface part wasLiNi_(0.77)Co_(0.11)Mn_(0.12)O₂ (Number in Table 1: 12-5 to 13), and aconcentration gradient layer was formed between the core part and thesurface part (see 12 to 12-5 in Table 1) to have concentration gradientsof nickel and manganese were used. After preparing cathode slurry usingDenka black as a conductive material and PVDF as a binder so that a massratio of the lithium-metal oxide, the conductive material, and thebinder was 92:5:3, the cathode slurry was coated, dried, and pressed onan aluminum substrate, thereby manufacturing a cathode.

A concentration gradient of the lithium-metal oxide is as illustrated inthe following Table 1, and the concentration gradient layer andconcentration measurement positions are as illustrated in FIG. 1. Theconcentration measurement positions were set at an interval of 0.4 μmfrom the center of a lithium-metal oxide particle in which a distancebetween the center of the particle and a surface thereof was 4.8 μm, anda molar ratio of each metal contained in the lithium-metal oxide wasmeasured at the positions 1 to 12 from the real center of thelithium-metal oxide. In addition, between the positions 12 and 13, amolar ratio of each metal was measured at an interval of 0.04 μm (40nm). Positions at which the concentration of each metal was measured atthe interval of 0.04 μm (40 nm) between the positions 12 and 13 wererepresented by positions 12-1, 12-2, 12-3, 12-4, 12-5, 12-6, 12-7, 12-8,and 12-9.

TABLE 1 Metal Concentration Measurement Depending on Position from Corepart of Lithium-Metal Oxide particle to Surface Part Thereof PositionMolar Ratio Molar Ratio Molar Ratio (Number) of Ni of Co of Mn 1 0.8020.110 0.088 2 0.801 0.111 0.088 3 0.802 0.110 0.088 4 0.802 0.110 0.0885 0.803 0.111 0.086 6 0.802 0.110 0.088 7 0.802 0.110 0.088 8 0.8020.109 0.089 9 0.801 0.110 0.089 10  0.802 0.110 0.088 11  0.802 0.1080.090 12  0.800 0.110 0.090  12-1 0.794 0.110 0.096  12-2 0.789 0.1090.102  12-3 0.782 0.110 0.108  12-4 0.777 0.110 0.113  12-5 0.770 0.1100.120  12-6 0.771 0.110 0.119  12-7 0.770 0.110 0.120  12-8 0.769 0.1110.120  12-9 0.770 0.109 0.121 13  0.770 0.110 0.120

Manufacturing of Anode

An anode slurry containing 93 wt % of natural graphite as an anodeactive material, 5 wt % of KS6, which is a flake type conductivematerial, as a conductive material, 1 wt % of styrene-butadiene rubber(SBR) as a binder, and 1 wt % of carboxymethyl cellulose (CMC) as athickener was coated, dried, and pressed on a copper substrate, therebymanufacturing an anode.

Manufacturing of Battery

A battery was configured by notching and stacking the cathode and theanode at a suitable size and interposing a separator (polyethylene,thickness: 25 μm) between the cathode and the anode, and tap portions ofthe cathode and the anode were welded, respectively. An assembly of thewelded cathode/separator/anode was put into a pouch, and three surfacesof the pouch except for an electrolyte injection surface thereof weresealed. Here, portions in which the tap was positioned were included insealing sites. After injecting the electrolyte through the surface thatwas not sealed, the remaining surface was also sealed, and impregnationwas performed for 12 hours or more. As the electrolyte, an electrolyteobtained by preparing 1M LIPF₆ solution using a mixed solvent ofethylene carbonate (EC), ethylmethyl carbonate (EMC), and dimethylcarbonate (DMC) (volume ratio; 25/45/30 (EC:EMC:DMC)) and basicallyadding 1 wt % of vinylene carbonate (VC) was used, and an electrolyte towhich 1,3-propane sultone (PS) and 1,3-propene sultone (PRS) wereadditionally added was also used (Examples 1 to 24 and ComparativeExamples 3 to 26).

Thereafter, per-charging was performed for 36 hours using a current(2.5A) corresponding to 0.25C. After 1 hour, degassing was performed,acing was performed for 24 hours or more, and then, initial charging anddischarging was performed (charging condition: CC-CV 0.2C 4.2V 0.05CCUT-OFF, discharging condition: CC 0.2C 2.5V CUT-OFF). Then, standardcharging and discharging was performed. (charging condition: CC-CV 0.5 C4.2V 0.05C CUT-OFF, discharging condition: CC 0.5C 2.5V CUT-OFF).

Evaluation of Battery Characteristics

Power, high temperature storage characteristics, and cycle lifecharacteristics of the batteries manufactured in Examples 1 to 24 wereevaluated and illustrated in Table 2.

TABLE 2 Capacity Recovery Rate Cycle (%) after Life Keeping RetentionCathode Battery at Rate (%) Active PS PRS Power 60° C. for After 500Classification Material (wt %) (wt %) (W/kg) 4 Weeks Cycles Example 1CAM10 0.5 0 2469 73.4 85.0 Example 2 CAM10 1 0 2463 76.1 85.0 Example 3CAM10 1.5 0 2452 78.9 84.9 Example 4 CAM10 2 0 2438 80.7 84.9 Example 5CAM10 0 0.5 2461 74.6 84.8 Example 6 CAM10 0 1 2439 77.9 84.8 Example 7CAM10 0 1.5 2417 80.9 84.6 Example 8 CAM10 0 2 2403 85.4 84.8 Example 9CAM10 0.5 0.5 2449 77.1 84.9 Example CAM10 0.5 1 2432 80.2 85.0 10Example CAM10 0.5 1.5 2408 84.6 84.8 11 Example CAM10 0.5 2 2391 87 84.612 Example CAM10 1 0.5 2437 80.2 84.7 13 Example CAM10 1 1 2417 82.484.5 14 Example CAM10 1 1.5 2398 87.2 84.6 15 Example CAM10 1 2 237789.7 84.7 16 Example CAM10 1.5 0.5 2432 82.7 84.8 17 Example CAM10 1.5 12413 85.9 84.8 18 Example CAM10 1.5 1.5 2387 88.5 84.4 19 Example CAM101.5 2 2371 93.2 77.9 20 Example CAM10 2 0.5 2419 85.1 84.7 21 ExampleCAM10 2 1 2397 87.4 84.8 22 Example CAM10 2 1.5 2378 91.2 83.4 23Example CAM10 2 2 2361 95.6 76.9 24

In the batteries in Examples, even though PS, PRS, or mixture thereof,which is a sulfur based electrolyte additive, was contained, there wasalmost no decrease in power, and high-temperature storagecharacteristics and a cycle life retention rate were excellent.Particularly, in most cases, the cycle life retention rate was 80% ormore, such that cycle life characteristics were significantly excellent.

COMPARATIVE EXAMPLES 1 to 26

Batteries in Comparative Examples 3 to 26 were manufactured in the samemanner in Examples 1 to 24 except for using LiNi_(0.8)Co_(0.1)Mn_(0.102)(hereinafter, CAM20) having a uniform composition in an entire particleas a cathode active material. In Comparative Example 1, a battery wasmanufactured using CAM10, which is the cathode active material having aconcentration gradient, without adding PS or PRS as an additive. InComparative Example 2, a battery was manufactured using CAM20, which isthe cathode active material having a uniform composition in an entireparticle, without adding PS or PRS as an additive.

Power, high-temperature storage characteristics, and cycle lifecharacteristics of the batteries manufactured in Comparative Examples 1to 26 were evaluated and illustrated in Table 3.

TABLE 3 Capacity Recovery Cycle Rate (%) Life after Retention KeepingRate Cathode Battery at (%) After Active PS PRS Power 60° C. for 500Classification Material (wt %) (wt %) (W/kg) 4 Weeks Cycles ComparativeCAM10 0 0 2480 71 85 Example 1 Comparative CAM20 0 0 2500 71 68 Example2 Comparative CAM20 0.5 0 2473 73.1 67.8 Example 3 Comparative CAM20 1 02439 74.9 67.7 Example 4 Comparative CAM20 1.5 0 2412 76.7 67.6 Example5 Comparative CAM20 2 0 2378 79.4 67.1 Example 6 Comparative CAM20 0 0.52451 74.2 67.7 Example 7 Comparative CAM20 0 1 2397 76.5 67.4 Example 8Comparative CAM20 0 1.5 2348 79.7 67.2 Example 9 Comparative CAM20 0 22299 83.4 66.6 Example 10 Comparative CAM20 0.5 0.5 2423 75.9 67.5Example 11 Comparative CAM20 0.5 1 2372 79.1 67.2 Example 12 ComparativeCAM20 0.5 1.5 2319 81.9 66.9 Example 13 Comparative CAM20 0.5 2 227185.7 62.9 Example 14 Comparative CAM20 1 0.5 2393 77.7 67.4 Example 15Comparative CAM20 1 1 2342 81.4 67 Example 16 Comparative CAM20 1 1.52288 83.5 60.7 Example 17 Comparative CAM20 1 2 2239 86.4 56.1 Example18 Comparative CAM20 1.5 0.5 2361 79.7 67.6 Example 19 Comparative CAM201.5 1 2313 83.4 60.3 Example 20 Comparative CAM20 1.5 1.5 2257 86.1 59.5Example 21 Comparative CAM20 1.5 2 2212 88.5 51.6 Example 22 ComparativeCAM20 2 0.5 2327 81.7 64.3 Example 23 Comparative CAM20 2 1 2278 85.460.2 Example 24 Comparative CAM20 2 1.5 2231 87.7 57.2 Example 25Comparative CAM20 2 2 2177 91.7 48.5 Example 26

Referring to Tables 2 and 3, in Examples in which the cathode activematerial having a concentration gradient formed between the surface partand the core part was contained, high-temperature storagecharacteristics and power characteristics were similar to those inComparative Examples, but cycle life characteristics were significantlyexcellent.

It may be appreciated that in the case of using CAM20 corresponding tothe cathode active material not having the concentration gradient andusing PS or PRS corresponding to electrolyte additive, high-temperaturestorage characteristics were improved, but power was decreased, andcycle life characteristics were deteriorated. Particularly, in all ofthe batteries in Comparative Examples, a cycle life retention rate wasless than 70% due to deterioration of cycle life characteristics. Thatis, the existing problems occurring in the case of using the sulfurbased additive still remained.

On the contrary, it may be confirmed that in the case of using CAM10corresponding to the cathode active material having the concentrationgradient and using PS or PRS corresponding to the sulfur basedelectrolyte additive, high-temperature storage characteristics weresignificantly improved, but decreases in power and cycle life were notlarge as, compared to the case of using CAM20. Therefore, in the case ofusing CAM10 corresponding to the cathode active material containing thelithium-metal oxide having a concentration gradient, even in the case ofusing PS or PRS causing a trade-off such as decreases in power and cyclelife, it is possible to manufacture a significantly excellent lithiumsecondary battery of which high-temperature storage characteristics wereexcellent and in which there was almost no trade-off such as decreasesin power and cycle life.

According to the present invention, it is possible to manufacture thelithium secondary battery having significantly excellenthigh-temperature storage characteristics while minimizing decreases inpower and cycle life occurring in the battery using the existing sulfurbased additive by using the concentration gradient type cathode activematerial as the cathode material.

What is claimed is:
 1. A lithium secondary battery comprising a cathode,an anode, and a non-aqueous electrolyte, wherein the non-aqueouselectrolyte contains a sulfur based additive, and the cathode contains acathode active material containing a lithium-metal oxide particle, thelithium-metal oxide particle including a core part represented byChemical Formula 5 and a surface part represented by Chemical Formula 6,having a concentration gradient layer of Ni in which a concentration ofNi is decreased in a surface direction between the surface part and thecore part, containing a constant concentration of Co between the surfacepart and the core part, and having a concentration gradient layer of Mnin which a concentration of Mn is increased in the surface directionbetween the surface part and the core part:Li_(x4)Ni_(a4)Co_(b4)Mn_(c4)O_(y4)   [Chemical Formula 5]Li_(x5)Ni_(a5)Co_(b5)Mn_(c5)O_(y5)   [Chemical Formula 6] (in ChemicalFormulas 5 and 6, x4, x5, y4, y5, a4, a5, b4, b5, c4, and c5 satisfy0<x4≤1.1, 0<x5≤1.1, 2≤y4≤2.02, 2≤y5≤2.02, 0.800≤a4≤1.000, 0≤a5≤0.770,0<b4≤0.11, 0<b5≤0.11, 0≤c4≤0.090, 0.120≤c5≤1.000, 0<a4+b4+c4≤1, and0<a5+b5+c5≤1), wherein the core part has a radius of equal to or greaterthan 0.6 μm from the center of the lithium-metal oxide particle, and theconcentration gradient layer has a width of equal to or greater than 0.2μm, and wherein the lithium-metal oxide particle is doped with at leastone metal ingredient selected from the group consisting of Al, Ti, Ba,Zr, Si, B, Mg, P, V, and a combination thereof.
 2. A lithium secondarybattery comprising a cathode, an anode, and a non-aqueous electrolyte,wherein the non-aqueous electrolyte contains a sulfur based additive,and the cathode contains a cathode active material containing alithium-metal oxide particle, the lithium-metal oxide particle includinga core part represented by Chemical Formula 5 and a surface partrepresented by Chemical Formula 6, having a concentration gradient layerof Ni in which a concentration of Ni is decreased in a surface directionbetween the surface part and the core part, containing a constantconcentration of Co between the surface part and the core part, andhaving a concentration gradient layer of Mn in which a concentration ofMn is increased in the surface direction between the surface part andthe core part:Li_(x4)Ni_(a4)Co_(b4)Mn_(c4)O_(y4)   [Chemical Formula 5]Li_(x5)Ni_(a5)Co_(b5)Mn_(c5)O_(y5)   [Chemical Formula 6] (in ChemicalFormulas 5 and 6, x4, x5, y4, y5, a4, a5, b4, b5, c4, and c5 satisfy0<x4≤1.1, 0<x5≤1.1, 2≤y4≤2.02, 2≤y5≤2.02, 0.800≤a4≤1.000, 0≤a5≤0.770,0<b4≤0.11, 0<b5≤0.11, 0≤c4≤0.090, 0.120≤c5≤1.000, 0<a4+b4+c4≤1, and0<a5+b5+c5≤1), wherein the core part has a radius of equal to or greaterthan 0.6 μm from the center of the lithium-metal oxide particle, and theconcentration gradient layer has a width of equal to or greater than 0.2μm, and wherein the lithium-metal oxide particle further includes acoating layer formed of Al, Ti, Ba, Zr, Si, B, Mg, P, an alloy thereof,or an oxide thereof.
 3. A lithium secondary battery comprising acathode, an anode, and a non-aqueous electrolyte, wherein thenon-aqueous electrolyte contains a sulfur based additive, wherein thesulfur based additive is a sultone based additive, and the cathodecontains a cathode active material containing a lithium-metal oxideparticle, the lithium-metal oxide particle including a core partrepresented by Chemical Formula 5 and a surface part represented byChemical Formula 6, having a concentration gradient layer of Ni in whicha concentration of Ni is decreased in a surface direction between thesurface part and the core part, containing a constant concentration ofCo between the surface part and the core part, and having aconcentration gradient layer of Mn in which a concentration of Mn isincreased in the surface direction between the surface part and the corepart:Li_(x4)Ni_(a4)Co_(b4)Mn_(c4)O_(y4)   [Chemical Formula 5]Li_(x5)Ni_(a5)Co_(b5)Mn_(c5)O_(y5)   [Chemical Formula 6] (in ChemicalFormulas 5 and 6, x4, x5, y4, y5, a4, a5, b4, b5, c4, and c5 satisfy0<x4≤1.1, 0<x5≤1.1, 2≤y4≤2.02, 2≤y5≤2.02, 0.800≤a4≤1.000, 0≤a5≤0.770,0<b4≤0.11, 0<b5≤0.11, 0≤c4≤0.090, 0.120≤c5≤1.000, 0<a4+b4+c4≤1, and0<a5+b5+c5≤1), wherein the core part has a radius of equal to or greaterthan 0.6 μm from the center of the lithium-metal oxide particle, and theconcentration gradient layer has a width of equal to or greater than 0.2μm.
 4. The lithium secondary battery of claim 3, wherein thelithium-metal oxide particle is doped with at least one metal ingredientselected from the group consisting of Al, Ti, Ba, Zr, Si, B, Mg, P, V,and a combination thereof.
 5. The lithium secondary battery of claim 3,wherein the lithium-metal oxide particle further includes a coatinglayer formed of Al, Ti, Ba, Zr, Si, B, Mg, P, an alloy thereof, or anoxide thereof.